Practical Presentation: Study of Linkage Maps Based on Data from Drosophila Crosses

 Practical Presentation: Study of Linkage Maps Based on Data from Drosophila Crosses

 

Analyzing Gene Linkage and Mapping in Drosophila melanogaster 

 

 Introduction to Linkage Maps

Linkage maps are representations of the relative positions of genes on a chromosome, based on recombination frequencies.

Purpose:  Used to study how genes are inherited together.

Genetic Linkage: Genes located close together on the same chromosome are likely to be inherited together.

Recombination: Crossing over during meiosis can separate linked genes.

Recombination Frequency: Used to estimate the distance between genes (measured in map units or centimorgans).

 

 Drosophila Crosses Setup

Parental Traits:

 Parent 1: Normal wings and red eyes (dominant).

Parent 2: Vestigial wings and white eyes (recessive).

 

F1 Generation: Offspring show dominant traits (normal wings, red eyes).

 

Test Cross: F1 offspring crossed with homozygous recessive flies (vestigial wings, white eyes).

  

Generation Phenotypes

Expected Phenotypes in the F2 Generation:

Parental types (e.g., normal wings, red eyes).

Recombinant types (e.g., normal wings, white eyes; vestigial wings, red eyes).

 

Observation: The number of recombinants indicates how often crossing over occurs between the linked genes.

 

Calculating Recombination Frequency

Recombination Frequency Formula:

Example:

 Total Offspring: 1000

 Recombinants: 150

 Recombination Frequency = (150 / 1000) × 100 = 15%

 Interpretation: The genes are 15 centimorgans apart.

 

Constructing the Linkage Map

Process:

1)    Collect recombination frequencies between multiple genes.

2)    Arrange genes on the chromosome based on these distances.

 

Example of  Map:

·          Gene Pair 1: Wing Shape (A) and Eye Color (B)

• Recombination Frequency: 15% → Distance = 15 cM

 

·         Gene Pair 2: Eye Color (B) and Body Color (C)

• Recombination Frequency: 10% → Distance = 10 cM

 

·         Gene Pair 3: Wing Shape (A) and Body Color (C)

• Recombination Frequency: 25% → Distance = 25 cM

 

With these distances, you can infer the order of the genes:

• Wing Shape (A) → Eye Color (B): 15 cM
• Eye Color (B) → Body Color (C): 10 cM
• The total distance between Wing Shape (A) and Body Color (C) is 25 cM.

Thus, the order of genes on the chromosome is likely to be:

$$A (Wing Shape)−15cM−B (Eye Color)−10cM−C (Body Color)$$

Map Construction: Based on relative distances.

Interference and Coincidence

Interference: A crossover event in one region of a chromosome can reduce the probability of another crossover occurring nearby.

Effect on Mapping: Affects recombination frequencies, so adjustments may be needed to refine maps.

 

Practical Applications of Linkage Maps

1.  Understanding gene inheritance patterns.

2.  Used in agriculture and animal breeding for trait selection.

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3.  Helps in studying genetic diseases and traits.